1. Field of the Invention
The present invention relates generally to composite materials and methods for bonding various layers of composite assemblies together. More particularly, the present invention involves promoting adhesion between the structural portion of a composite assembly and the exterior portion of the assembly whether it is a gel coating or other substrate.
2. Description of Related Art
Composite materials include a combination of resin matrix and fiber reinforcement. Composite materials have been developed for use in wide variety of applications that include a vast array of resin/fiber combinations and orientations. A common type of composite assembly includes an underlying structural composite component that is covered with a surface finish. The structural component typically includes a resin matrix in which multiple layers (plies) of fibrous reinforcement are located. Surface finishes include a variety of resin coatings, foam, wood, metal and plastic laminates.
Pre-impregnated composite material (prepreg) is used widely in the manufacture of composite parts. Prepreg is a combination of uncured resin matrix and fiber reinforcement, which is in a form that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired. It is well known that the relative amount of fibers and resin in a composite part and the distribution of resin within the fiber network have a large affect on the structural properties of the part. Prepreg is a preferred material for use in manufacturing structural parts where it is important that the part meets established strength and weight requirements.
Gel coating is a popular surface coating technique in which a resin is typically applied to the mold surface and partially cured to a gelled state. Resin and fibrous support are then layered onto the gel coat or applied as a prepreg. The resulting assembly is then cured to form the composite part. Gel coats have been widely used in the marine industry for the production of large parts, such as boat hulls and masts and other marine parts that require a smooth and rugged surface finish. Gel coats have also been used in the production of other large parts, such a wind turbine blades where large size, high strength and low weight are a desired combination.
The resin matrices commonly used for the above mentioned large structures are formulated to cure at temperatures of around 120° C. These resins are frequently supplied in the form of a prepreg that is composed of about 60 to 70 weight % reinforcing fiber, such as glass fibers or carbon fibers and about 30 to 40 weight % of a thermosetting resin matrix, such as epoxy. As part size increases, the curing temperature becomes an important consideration. Large amounts of energy are required and the cost of constructing sufficiently large ovens with the required high temperature capability is not economically attractive. In addition, the tooling used for molding composite parts at cure temperatures of 120° C. and above must be made from expensive premium materials that can become extremely costly as the size of the part being molded increases.
One approach to reducing molding costs for large parts has been to move the curing temperature below 100° C. Target curing temperatures of below 90° C. have been particularly attractive. For boat hulls and other large structures, maximum curing temperatures on the order of around 65° C. and lower have been tried. Unfortunately, the reduction in cure temperature also causes reduced adhesion between the gel coating and underlying composite material regardless of how long the assembly is cured.
One approach to increasing gel coat adhesion at low curing temperatures has been to increase the amount of curing agent in the resin matrix of the uncured composite material. This approach has provided some increase in adhesion. However, the room temperature handling time (also referred to as out-life) of the resin or prepreg is severely reduced when extra curing agent is added. In addition, the tack (stickiness) of the resin or prepreg drops relatively quickly at room temperature when extra curing agent is used. Good tack is required in order for the uncured composite material to stick to the gel coat and the other laminates in the mold as the assembly is formed.
Another approach to increasing gel coat adhesion at low curing temperatures has involved placing an adhesive between the composite material (prepreg) and the gel coat. These adhesive layers are referred to as “tie coats” and typically are made up of two parts. The first part is spread onto the surface of the gel coat after it has gelled and the second part is added shortly before the composite material (prepreg) is laid up in the gel-coated mold. This approach is not entirely satisfactory, since it is difficult to apply an even coating of the two adhesive parts. In addition, there are environmental concerns, such as amine corrosion and amine sensitivity to moisture/carbon dioxide for epoxy-based gel coats and styrene emission concerns for polyester gel coat systems.
In view of the above, there is a continuing need to develop new processes and systems for making composite assemblies at low cure temperatures where the adhesion between the underlying composite material and exterior finish coating is increased.